A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In stage systems of a lithographic apparatus, for instance in a substrate or wafer stage, leaf springs are used to mount one object of the stage to another object of the stage. In particular leaf springs are used to mount a magnet yoke, which is part of a motor of the stage, in a mirror block of the wafer stage. The leaf springs are used to minimize over determined fixation of the minor block with respect to the magnet yoke. The leaf spring has a plane-shaped body which provides a relative high stiffness in the plane of the body. By providing three leaf springs in different orientations between the yoke and the mirror block, the motor can be mounted stiff in six degrees of freedom in the mirror block.
To avoid an over determined fixation the leaf spring should provide relative stiff connection in two orthogonal directions, while the leaf spring should provide very low stiffness in all other degrees of freedom. In this way deformation as a consequence of manufacturing errors, difference in coefficient of thermal expansion in the motor and minor block, possible hysteresis in serial connections during large crash loads, etc may be avoided.
The leaf spring is connected to the minor block using glue. As a consequence, the choice of material for the leaf spring is mainly determined by the coefficient of thermal expansion of the material. This results for instance in a choice of Invar as material for the leaf spring. However, other material properties of Invar determining the achievable stiffness ratios between the different directions are relatively poor.
With this known construction it is not possible to achieve the desired high stiffness in two orthogonal directions, while at the same time the stiffness in other degrees of freedom is weak enough. Thus, the construction of the leaf spring is still over determined.